The paper industry has been practicing waste paper recycling to regenerate usable cellulosic fiber for paper making for many decades. In these processes, ink is removed from the waste paper pulp using a suitable deinking chemical composition. By controlling the deinking process, a recycling mill can affect the properties of the paper such as the brightness and can improve the usability of the cellulosic fiber for paper manufacturing.
Deinking consists of a series of complex chemical and physical processes. These events include but are not limited to ink detachment, ink dispersion, ink collection, ink transport, and removal of inks from the waste paper pulp slurry. Each of these microprocesses have different surface and interfacial demands within the recycling operation in order to efficiently and effectively deink waste paper and produce quality paper.
Conventionally, two different methods have been employed to remove the ink and ink related entities like specks (agglomerates of ink particles) in order to produce the deinked fiber after repulping. These two processes are flotation and wash deinking. Often, processes contain both flotation and wash deinking and can be referred to as combination deinking processes. The underlying chemical and physical requirements to successfully deink are different for flotation, wash, and combination deinking processes, and are also strongly dependant of the waste paper composition.
More specifically, flotation/washing combination deinking refers to a deinking process wherein the ink, detached from the paper pulp by the deinking chemical composition and the action of mechanical energy in the pulping step, is separated from the cellulosic fibers primarily through the flotation devices or flotation cells of the recycling process prior to passing the pulp through washing devices in the washing stage.
Flotation processes are fundamentally different than washing processes. This difference is partly because the ink particle size and hydrophobicity are important for favorable separation. Flotation methods of ink removal generally involve passing air bubbles through an aqueous system containing dispersed cellulose fiber and mineral filler that are most often produced via a repulping process. The resulting repulped slurry of cellulose fiber and mineral filler having therein additives added either before, during, or after repulping. As the air bubbles rise within the slurry and carry the ink particles with them through specific attractive interaction, they generate foam which is enriched in ink that is subsequently removed from the top of the flotation cell. The amount of foam that is typically favored in flotation deinking systems is an amount which can be collected by skimming, aspiration, or other means, and which transports with it an enriched concentration of ink while minimizing the amount of other solids like fibers and fillers that are rejected.
Typically, either a flotation-derived or flotation/washing-derived deinking chemistry is utilized in a given deinking system. At some point in either process, the deinked, repulped waste paper is often passed through a series of fine cleaners and/or screens where the small particulate contaminants (e.g., sand and grit) are removed. Additional processing stages may be required such as, for example, dispersion, to reduce the particle size of any contaminants, or a special cleaning stage with special cleaners designed to remove specific contaminants.
The deinked waste paper is then held in storage until it is eventually fed to a papermaking machine.
The chemistry used in traditional deinking involves most often addition of fatty acid soaps that are efficient ink collectors for ink flotation in alkaline systems (pH greater than 9), although these soaps can demonstrate decreased ink detachment characteristics and lead to deposit concerns later in the process.
Surfactant-based deinking aids, especially specific nonionic surfactants, can be excellent for ink detachment and ink dispersion and transport. Some nonionic surfactants assist in ink collection. If not properly chosen, however, these additives can also actually hinder ink collection in flotation processes. Flotation deinking usually utilizes different surfactants than washing because the resulting surface properties and size of the ink particles that is beneficial for flotation deinking is different than is desirable for wash deinking. Examples of traditional nonionic surfactants that may be used in the flotation deinking process include alkylene oxide adducts of fatty alcohols or fatty acid, and alkanolamides. Usually, nonionic surfactants are used in association with soap, either separately or in combined blend of nonalkoxylated fatty acid soap and nonionic surfactant.
The chemistry involved also in traditional deinking very often involves addition of caustic soda in the pulper to increase the pH, often greater than 9 and sometimes greater than 10. Increasing the pH, however, often causes yellowing and darkening of the waste paper stock—especially when the waste paper contains groundwood or mechanical pulps. To counteract this undesirable darkening effect, a bleaching additive is typically added to increase the whiteness and brightness of the pulp. Sodium silicate, or waterglass, is also of common use in alkaline condition to protect bleaching additives from chemical decomposition by metallic ions, and to help in ink collection.
In addition to final deinked pulp quality, productivity of the deinking process is a key factor of cost-effective deinked pulp production. Total yield of deinking processes range typically from 90% down to 60%. In these 10% to 40% losses, the hydrophobic contaminant including ink is only about a few percent, typically less than 4%. So the main losses in deinking of recovered printed paper are fibers and mineral fillers, and these losses are inherent in any flotation or flotation/washing deinking processes.
In flotation deinking processes, the flotation step is the main source of losses. High Brightness gain at the flotation step with respect to high flotation yield is the key characteristic of a cost-effective flotation deinking process.
WO 2007/081921 discloses a method of deinking printed waste paper, comprising the steps of converting the waste paper to an aqueous pulp slurry in a pulper, contacting the aqueous pulp slurry with inorganic particles and with a deinking composition comprising a non-ionic surfactant or a fatty acid, and recovering deinked paper pulp from the aqueous pulp slurry. The inorganic particles may be hydrophobically-modified inorganic particles, and are preferably calcium carbonate.
EP 0 989 229 B1 discloses a process for eliminating and inhibiting adhesive impurities, i.e. so called stickies, from paper pulp during the process of treating waste paper. During the treatment of the waste paper a hydrophobized synthetic or natural mineral (such as zeolite or calcium carbonate) is added to the paper pulp prior to or during supplying the paper pulp into the flotation installation. The mineral is so strongly hydrophobized that it is not dispersible in water.
US 2007/0284067 discloses a composition for deinking waste paper, comprising a surfactant and a mineral-based deinking component. The mineral-based deinking component is hydrophobic and has been prepared by treating the mineral particles with hydrophobizing reagents. An exemplified hydrophobized mineral is kaolin hydrophobized with alkyl hydroxamate and tall oil, the latter being activated by calcium chloride. Also other minerals are mentioned such as talc, quartz, mica, potash, kyanite etc.
U.S. Pat. No. 4,443,357 discloses hydrophobic silica or silicate which is the reaction product of hydrophilic silica or silicate and a hydrophobic higher-aliphatic alcohol, the reaction being carried out in a non-aqueous liquid carrier at a temperature of above 100° C. A preferred alcohol is a straight-chain primary alkanol having from 8 to 30 carbon atoms. According to this patent it is believed that the silanol groups on the surface of the silica or silicate react with the hydrophobic alcohol to form a stable chemical bond. The hydrophobic silica or silicate is used in liquid defoamer formulations, through dispersion in a liquid hydrocarbon carrier. Such compositions are used for reducing foam in black liquor.
In the description of the prior art in U.S. Pat. No. 4,443,357 it is set forth that for well over three decades, tiny particles of silica or relatively high-silica silicates have been used as dispersed additives in organic liquid carrier to make defoaming liquid composition for foam inhibition in aqueous foaming systems. Typically, these silicas or silicates have been treated to make them hydrophobic, perhaps the most common treating agent being silicone (organically substituted polysiloxane) oil. Furthermore, it is stated that the combination of silicone oil/hydrophilic silica at normally used ratios is ineffective as an antifoam unless it is heat treated at 150° C. for ca. 2 h. Under these conditions the silicon oil presumably reacts with the silica surface. In the description of the prior art in U.S. Pat. No. 4,443,357, it is stated also that the higher the degree of hydrophobicity of the modified silica particle, the higher its dispersability in the oily carrier and the higher its contribution to the efficiency of the defoaming liquid composition.
JP 2007-253014 discloses a powder defoaming agent comprising hydrophobic silica dispersed In a hydrocarbon oil and a porous powder which is hydrophilic silica, aluminium oxide, titanium oxide, calcium carbonate, magnesium carbonate, carbon black or talc. The defoaming agent is prepared by adsorbing the dispersion of hydrophobic silica in the hydrocarbon oil on the porous powder. The final powder defoaming agent is suitable for building materials, especially mortar and for plasters.
WO 91/05905 discloses aqueous dispersions of alkaline earth soaps and/or alkaline earth resin soaps for de-inking printed waste paper, which dispersions as a component may contain an alkali or alkaline earth metal aluminium silicate . The deinking composition Is In the form of an aqueous dispersion. According to the patentees, the dispersion gave equivalent deinking results compared to a commercially available dispersion containing calcium soap, and the main advantage of the dispersion was that the froth stability was considerably lower which eliminates the need of using foam inhibitors (page 3, line 25 - page 4, line 4, and page 22 last paragraph).
U.S. Pat. No. 4,231,841 discloses a process for the de-Inking of printed waste paper by using a de-Inking composition comprising five components which are:
(F) an alkali metal salt of a fatty acid,
(G) a non-ionic surfactant,
(H) an anionic surfactant,
(I) sodium- carboxylmethyl-cellulose, and
(J) an alkaline inorganic salt selected from the class consisting of metasilicate, disilicate, carbonate, borate and polyphosphate, especially sodium metasilicate. An object of the deinking composition was to produce a pulp having a high brightness and with a high yield during the recovery step.
WO 02/12618 discloses a flotation deinking process comprising adding to an aqueous waste paper pulp slurry (1) a polyester, (2) a first deinking agent which Is an alkoxylated hydrophobic base and (3) a second deinking agent which is an alkoxyiated hydrophobic base having a lower cloud point than the first deinking agent (claim 1). D4 also mentions that typical chemicals used in the pulper Include NaOH and hydrogen peroxide and that sodium silicate, a metal chelating agent and calcium chloride may also be added to the pulper and/or flotation to maintain water hardness to ensure calcium soap formation.
In flotation or flotation/washing deinking process of printed waste paper, there is a need for a more efficient and cost effective chemical solution to reduce the loss of fibers and fillers in the flotation step while keeping an effective ink collection and so a deinked pulp production of high quality, i.e. having both high brightness and low residual concentration of ink and ink related entitles like specks.